JP-2009-534007A (WO 2007/117473, US 2009/0235766) describes a valve control device including a valve drive unit and a rotation angle detector. The valve drive unit activates a valve stem (shaft) of a poppet valve to reciprocate in the axial direction (stroke direction) so as to adjust a flow rate of exhaust gas. The rotation angle detector detects an actual opening of a valve by measuring a rotation angle of an output gear. The valve control device controls a motor so that the actual opening of the valve detected by the rotation angle detector is controlled to be equal to a target value.
The valve drive unit includes an actuator which has the motor as source of power, a deceleration mechanism which slows down rotations of the motor by two steps, and a spring which generates elastic power biasing the poppet valve to return from a valve open position to a full close position.
The deceleration mechanism has a pinion gear, a middle gear in addition to the output gear. The pinion gear is fixed to an output shaft of the motor. The middle gear is rotated by engaging with the pinion gear. The output gear is rotated by engaging with the middle gear. The output gear rotates around an output gear shaft provided to an actuator housing. Moreover, the output gear integrally has a cam slot which changes the rotary motion of the actuator into the rectilinear motion of the valve stem. The cam slot has a groove shape corresponding to the operation pattern of the poppet valve.
The cam slot of the output gear is coupled with a bearing attached to an input unit of the valve stem by a pin inserted into the cam slot. Moreover, the poppet valve is combined with an output unit of the valve stem. Furthermore, the cam slot has a cam full close stopper which regulates the rotation of the output gear by colliding with the bearing at a cam full close position, when the output gear rotates to exceed the full close position of the poppet valve.
In the valve control device, the output gear and the cam slot are rotated by the torque of the motor. Thus, the bearing, the pin, the valve stem and the poppet valve are moved to reciprocate in the axial direction of the valve stem, such that the poppet valve is seated on or lifted from the valve seat which defines a valve full close position.
Moreover, the rotation angle detector has a rotation angle sensor which outputs a sensor signal corresponding to the rotation angle of the output gear as a cam rotation angle to an electronic control unit. As shown in FIG. 8, the sensor output (voltage) characteristics are set with respect to the cam rotation angle by two points that are the valve full open position J2 and the valve full close position J1 (at which the flow rate is zero).
That is, in the characteristic line (i.e., the sensor output characteristics line with respect to the cam rotation angle) shown in the lower graph of FIG. 8, the sensor output is written at the valve full close position J1 when the poppet valve is fully closed, and is written at the valve full open position J2 when the poppet valve is fully opened.
However, the cam full close position is unclear (different among EGR control valves) with respect to the valve full close position, due to a dimension R0. Therefore, when the poppet valve is seated on the valve seat to be held at the valve full close position, that is when the poppet valve is controlled to be fully closed, the sensor output (voltage) may be varied with respect to the cam full close position.
By this reason, the poppet valve may overshoot the target position when the poppet valve is controlled from the valve open position to the valve full close position. At this time, the bearing may contact to the cam full close stopper. In this case, the valve drive unit such as the gear, the cam and the motor may be deformed or damaged, so that the durability may be lowered.